os: fix LinkError creation on windows.

Not only carrying invalid info but also this caused Error to crash with
null pointer exception.

Change-Id: Ibfe63d20eb9b9178ea618e59c74111e9245a6779
Reviewed-on: https://go-review.googlesource.com/6270
Run-TryBot: Hyang-Ah Hana Kim <hyangah@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/os/file_windows.go

@@ -508,9 +508,8 @@ func Link(oldname, newname string) error {
 	if err != nil {
 		return &LinkError{"link", oldname, newname, err}
 	}
-
-	e := syscall.CreateHardLink(n, o, 0)
-	if e != nil {
+	err = syscall.CreateHardLink(n, o, 0)
+	if err != nil {
 		return &LinkError{"link", oldname, newname, err}
 	}
 	return nil

src/os/os_test.go

@@ -533,6 +533,14 @@ func TestHardLink(t *testing.T) {
 	if err != nil {
 		t.Fatalf("link %q, %q failed: %v", to, from, err)
 	}
+
+	none := "hardlinktestnone"
+	err = Link(none, none)
+	// Check the returned error is well-formed.
+	if lerr, ok := err.(*LinkError); !ok || lerr.Error() == "" {
+		t.Errorf("link %q, %q failed to return a valid error", none, none)
+	}
+
 	defer Remove(from)
 	tostat, err := Stat(to)
 	if err != nil {